Low vulnerability gun propellant

ABSTRACT

A prepolymer for use in preparing a binder for low vulnerability gun propellants. The condensation of an alkynediol and formaldehyde produces a prepolymer which is cured with a polyisocyanate. The resulting binder can be combined with 1,3,5,7-tetramethylenetetranitramine (HMX) or 1,3,5-trimethylenetrinitramine (RDX), and is readily extruded as multi-perforated propellant grains, the extruded material having a greatly reduced tendency to slumping, and an enhanced mass impetus value.

The invention described herein may be manufactured, used and licensed byor for the Government for governmental purposes without the payment ofany royalty thereon, and was made in the course of or under a contractwith the U.S. Department of the Army.

This application is a division of application Ser. No. 484,001, filedApr. 11, 1983, now U.S. Pat. No. 4,456,493.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ammunition, and more particularly tolow vulnerability gun propellants and novel polymers for use therein.

2. Description of the Prior Art

A continuing objective in the design of gun propellants is to provide agun propellant which is energetic when deliberately ignited, but whichexhibits high resistance to accidental ignition from heat, flame,impact, friction, and chemical action. Propellants possessing suchresistance to accidental ignition are known as "low vulnerability"(often abbreviated as LOVA) gun propellants.

LOVA gun propellants are well-known in the prior art. One approach forproducing low vulnerability gun propellants has been the use ofpolyurethane binders. For example, U.S. Pat. No. 4,091,729, to Bell etal, describes a main propellant charge which includes 25% by weight of1,3,5,7-tetramethylenetetranitramine (HMX) or1,3,5-trimethylenetrinitramine (RDX), in combination with 75% by weightof polyurethane binder. The polyurethane binder comprises 11.867% byweight of a hydroxyl-terminated block copolymer of propylene oxide andethylene oxide, 3.167% by weight of trimethylol propane, 9.967% byweight of lysine diisocyanate methyl ester, and 0.025% by weight oftitanyl acetyl acetonate. After mixing, the propellant is extruded andplaced in a curing oven in the form of small diameter tubes.

However, the propellants of the prior art suffer from the disadvantagesthat they are not readily extruded with multiple perforations, and theirmass impetus is only about 336,000 ft-lbf/lbm. While the mass impetus ofsuch a prior art propellant is acceptable, a higher mass impetus isdesirable. In addition, when extruded, a polyurethane such as thatdescribed in U.S. Pat. No. 4,091,729 can slump prior to coolingresulting in the partial closure of any perforations. "Slumping" is thetendency of an extruded material to flatten out rather than retainingits shape when placed on a flat surface while cooling and solidifying.Thus if the propellant as extruded is round in cross section, thematerial will tend to flatten out against the supporting surface.

Accordingly, a need exists for a polymer useful in making a lowvulnerability gun propellant which exhibits a desirably high massimpetus value and lends itself to the extrusion of multi-perforatedpropellant grains which will retain their extruded shape withoutslumping.

SUMMARY OF THE INVENTION

In one embodiment, the present invention includes a polyisocyanatecurable prepolymer comprising a polyalkynediol formal having a numberaverage molecular weight greater than 1000.

In another embodiment, the present invention includes a lowvulnerability gun propellant binder comprising the reaction product ofan alkynediol formal prepolymer having a number average molecular weightof greater than 1000 and a polyisocyanate curing agent.

In yet another embodiment, the present invention includes a lowvulnerability gun propellant comprising 75-85% by weight of particles of1,3,5-trimethylenetrinitramine or 1,3,5,7-tetramethylenetetranitramine,and 15-25% by weight of the binder which comprises another embodiment ofthe present invention.

In still another embodiment, the present invention includes a method ofpreparing the polyisocyanate curable prepolymer of the present inventioncomprising (a) reacting substantially equimolar amounts of formaldehydeand alkynediol while removing produced water until the reaction issubstantially complete, (b) adding to the product of step (a) anadditional between about 5 and about 20% by weight of formaldehyde or aformaldehyde source based upon the weight of formaldehyde initiallyreacted, and (c) reacting the additional formaldehyde while removingadditionally produced water until the reaction is substantiallycomplete.

The carbon-carbon triple bonds of the binder of the present inventionprovide additional energy when the propellant is ignited, yet arerelatively inert and therefore do not appreciably affect the desirablelow vulnerability of the LOVA gun propellant of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The polyurethane binder of the present invention is based upon a novelprepolymer prepared by the acid-catalyzed condensation of an alkynedioland formaldehyde.

Typical alkynediols which may be used in the present invention include2-pentyne-1,5-diol, 3-hexyne-1,6-diol, 4-octyne-1,8-diol, and5-decyne-1,10-diol. A preferred alkynediol for use in the presentinvention is 2-butyne-1,4-diol.

The formaldehyde may be added to the prepolymer reaction mixture in anysuitable manner well-known to those skilled in the art. However, it ispreferred for ease of manufacture that the formaldehyde be generated insitu from a formaldehyde source such as paraformaldehyde or symtrioxanedue to the presence of the acid catalyst.

Any of the acid catalysts well-known in the prior art for use in suchcondensation reactions can be used in the practice of the presentinvention. The preferred acid catalyst is p-toluene sulfonic acid. Othersuitable catalysts include sulfuric acid, hydrochloric acid,hydrofluoric acid, m-benzene disulfonic acid, zinc chloride, ferricchloride, and boron trifluoride. The acid catalyst is used in aneffective catalytic amount, typically from about 0.01 to about 1.0 wt.%based on the weight of formaldehyde, or formaldehyde source, e.g.,paraformaldehyde, or symtrioxane.

Suitable solvents for the use in preparing the prepolymer are well-knownto those skilled in the art, such as benzene or toluene. However, thereaction may also be conducted in other similar substantially inertsolvents.

As stated above, a particularly preferred combination of components forforming the prepolymer is 2-butyne-1,4-diol and formaldehyde. While aprepolymer containing carbon-carbon triple bonds is known in the priorart for use in rocket propellants, the prior art prepolymer is deficientin at least two respects. The prepolymer of the prior art has a lowmolecular weight (approximately 1000). In addition, the prior artprepolymer has a functionality of 2.0, thus requiring the use of across-linker such as trimethylol propane or DESMODUR N-100 (trademark ofFarbenfabriken Bayer AG for a triisocyanate for use in formingurethanes) which complicates its processing.

The present invention overcomes these deficiencies by providing asynthetic procedure for the preparation of prepolymer of a highermolecular weight and functionality than the prior art, thus avoiding theneed for a cross-linking agent in the production of the prepolymer. Theprepolymer of the present invention has a number average molecularweight greater than 1000, and preferably greater than about 1250.Furthermore, it has a functionality greater than 2.0.

The higher molecular weight and functionality are obtained by initiallyreacting the alkynediol and formaldehyde in approximately equimolaramounts, and then after the reaction is substantially complete, addingan additional 5-20 wt. % of formaldehyde compared to the amount offormaldehyde initially added and allowing the reaction to continue foran additional period of time. The resulting prepolymer has a desirablyhigher molecular weight and functionality than the prior art prepolymer.

The preferred procedure for producing the prepolymer of the presentinvention consists of charging a suitable reaction container with thealkynediol, paraformaldehyde or symtrioxane, a suitable acid catalyst,and a suitable solvent. The mixture is stirred at reflux for severalhours while continually removing the water produced by the condensationreaction. Additional paraformaldehyde or symtrioxane and acid catalystare added as the reaction proceeds while continuing to remove waterproduced by the condensation. After cooling, the solvent is decanted,and the polymer is melted. Water is then added and the mixture isstirred vigorously. Finally, stirring is stopped, the water decanted,and a suitable azeotropic solvent is added for use in the removal of anyremaining water by azeotropic distillation.

The resulting prepolymer may be cured with any suitable polyisocyanatewell known to those skilled in the art. Representative polyisocyanateswhich may be used in the practice of the present invention includedimethylene diisocyanate, propylene-1,2-diisocyanate, tetramethylenediisocyanate, pentamethylene diisocyanate, octamethylene diisocyanate,benzene-1,3,5-triisocyanate, benzene-1,2,4-triisocyanate,toluene-2,3,4-triisocyanate, and toluene-2,4,6-triisocyanate. Apreferred diisocyanate compound is hexamethylene diisocyanate.

The amount of polyisocyanate used depends on the desired hardness of thecured binder. The amount of polyisocyanate used is based upon theequivalents of isocyanate (NCO) to hydroxyl (OH). The ratio of NCO/OH istypically between about 0.85:1 and about 1.3:1. The greater the amountof NCO, the harder the cured binder. Preferably the ratio is about 1:1.

If desired, cross-linking agents well known in the art, such astrimethylol propane, can also be added in an effective amount, typicallyat between about 0.02 and about 1.0% by weight based on the weight ofprepolymer, to increase cross-linking.

To prepare a gun propellant according to the present invention, theprepolymer, a polyisocyanate, between about 75 and about 85% by weightof 1,3,5,7-tetramethylenetetranitramine (HMX) or1,3,5-trimethylenetrinitramine (RDX), and a cross-linking agent, ifused, are combined and mixed in any order in a suitable heated mixingapparatus used for forming propellants, at a temperature of betweenabout 140° and about 160° F.

The crystals of HMX or RDX preferably have a weight mean diameter ofabout two microns. One means for producing such a particle size is fluidenergy milling. A two micron weight mean diameter results in a burningrate exponent of 0.98 instead of the greater than one values obtainedwhen using larger particle sizes. However, the binder of the presentinvention can be used with larger particle sizes if desired.

The mixture is then extruded with multiple perforations in any ofseveral commercially available extruders at a temperature between about140° and about 160° F. and a pressure of about 700-1000 psi. Almostimmediately after extrusion the gun propellant cools and solidifies withvery little slumping to produce a LOVA gun propellant having a massimpetus of at least about 350,000 ft-lbf/lbm.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat these examples are intended only to be illustrative without servingas a limitation on the scope of the present invention.

EXAMPLE 1

A 22 liter three-necked flask, equipped with a mchanical stirrer, twoDean-Stark traps with condensers, and a heating mantle, is charged withtechnical grade butyne-1,4-diol (4300 g), paraformaldehyde (1500 g),p-toluene sulfonic acid (50 g) and toluene (15 l). The mixture isstirred at reflux for four hours, during which time water (about 900 ml)is removed.

Paraformaldehyde (150 g) and p-toluene sulfonic acid (5 g) are added andrefluxing is continued for three hours, during which time water (100 ml)is removed. After cooling, the solvent is decanted from the polymericprecipitate. The polymer is melted and hot water (5.5 l) is added. Themixture is stirred vigorously for 1/4 hour, then allowed to settle andthe water decanted. This washing is repeated twice.

Benzene (6 l) is added and the water removed by azeotropic distillation.The benzene is then distilled off and the last traces of solvents areremoved under vacuum. The polymer analyzes at 0.16 equivalents OH/100 g,M_(n) =1260. The melting point range is 55°-60° C., and the color ispale brown.

The prepolymer of Example 1, when cured with hexamethylene diisocyanateat an NCO/OH ratio of about 1:1, results in the elemental compositionand heat of combustion values given in Table I. The heat of combustionvalue can be used to calculate the heat of formation value shown inTable I.

                  TABLE I                                                         ______________________________________                                                           Elemental Composition, %                                                      C     H       N                                            ______________________________________                                                           59.79 6.305   2.097                                        Empirical Formula (100 g mole)                                                                     C.sub.4.983 H.sub.6.305 N.sub.0.1498 O.sub.1.988         Experimental Heat of Combustion =                                                                  6.44 kcal/g.                                             Heat of Formation =  -40.0 kcal/mole                                          ______________________________________                                    

The above heat of formation value can be used in making performancecalculations for various percentages by weight of HMX and the abovebinder. In Table II the HMX varies between 75% by weight and 85% byweight and the binder varies between 15% by weight and 25% by weight. Ascan be seen from Table II, the mass impetus values are considerablyhigher than those of the prior art.

                  TABLE II                                                        ______________________________________                                        Composition (Percent By Weight)                                               Binder     HMX          Mass Impetus (ft-lbf/lbm)                             ______________________________________                                        25         75           370,423                                               22.5       77.5         385,310                                               20         80           398,822                                               17.5       82.5         411,203                                               15         85           422,449                                               ______________________________________                                    

EXAMPLE 2

To prepare a gun propellant of the present invention, 22 grams of theprepolymer of Example 1 is added to 75 grams of crystalline HMX having atwo micron weight mean diameter, and 3 grams of hexamethylenediisocyanate (an NCO/OH equivalents ratio of about 1:1). The mix isheated to about 140° F. and mixed thoroughly in a Baker-Perkins UprightMixer.

The mixed composition is charged into an extruder heated at about 140°C., and approximately 700 psi of pressure is employed to extrude and cutthe propellant into a seven perforation, cured grain having a diameterof 0.224 inch and a length of 0.625 inch.

Almost immediately after extrusion, the solid propellant cools andsolidifies with very little slumping. Impulse bomb experiments on theresulting propellant provide the propellant performance values shown inTable III.

                  TABLE III                                                       ______________________________________                                        Mass (gm)                                                                             Pressure (psig)                                                                             Time (msec)                                                                              r.sub.b (in/sec)                             ______________________________________                                         2.400  1,690         71         .317                                          2.580  1,840         65         .346                                          5.150  3,960         32         .703                                         10.200  9,150         15         1.500                                        10.550  9,600         14         1.607                                        12.950  12,500        11         2.044                                        12.930  12,500        11         2.046                                        14.650  15,000         7         3.214                                        14.630  15,400         8         3.813                                        ______________________________________                                        Mass Impetus:     365,940 ft-lbf/lbm -Isochoric Flame Temperature: 2,626.d                      egree. K. (at 5,000 psia)                                   Linear Burning Rate Equation:                                                                   r.sub.b = 1.731 (.sup.--P/10,000).sup.0.982                 Burning Rate Exponent =                                                                         0.982                                                       Burning Rate =    1.731 in/sec.                                               Cut, 7 perforated grains:                                                                       diameter = 0.224 in;                                                          length = 0.625 in.                                          ______________________________________                                    

From the above calculations and test results it can be seen that the gunpropellants and binders of the present invention have performance valueswhich are significantly higher than the low vulnerability gunpropellants of the prior art and at least equivalent to conventionaldouble base (nitroglycerin/nitrocellulose) gun propellants.

While the invention has been described in terms of various preferredembodiments, one skilled in the art will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims.

What is claimed is:
 1. A polyisocyanate curable prepolymer comprising apolyalkynediol formal having a number average molecular weight greaterthan
 1000. 2. The prepolymer of claim 1 wherein the molecular weight isgreater than about
 1250. 3. The prepolymer of claim 1 wherein theprepolymer is formed from the condensation of formaldehyde and analkynediol comprising 2-butyne-1,4-diol, 2-pentyne-1,5-diol,3-hexyne-1,6-diol, 4-octyne-1,8-diol, or 5-decyne-1,10-diol.
 4. Theprepolymer of claim 1 wherein the prepolymer has a functionality ofgreater than 2.0.
 5. The prepolymer of claim 1 wherein thepolyalkynediol formal is poly-2-butyne-1,4-diol formal having a numberaverage molecular weight of
 1260. 6. The prepolymer of claim 5 having0.16 equivalents of OH/100 g of prepolymer.
 7. A method of preparing theprepolymer of claim 1 comprising:(a) reacting substantially equimolaramounts of formaldehyde and alkynediol while removing produced wateruntil the reaction is substantially complete, (b) adding to the productof step (a) an additional between about 5 and about 20% by weight offormaldehyde or a formaldehyde source based upon the weight offormaldehyde initially reacted, and (c) reacting the additionalformaldehyde while removing additionally produced water until thereaction is substantially complete.
 8. The method of claim 7 wherein theformaldehyde source is paraformaldehyde or symtrioxane.